Cathode-Induced Stress Can Suppress Dendritic Failure in Thin Solid Electrolyte Separators

Abstract Solid-state batteries pairing lithium metal anodes with inorganic solid electrolytes promise a step change in energy density, but during charging at practical rates lithium dendrites penetrate the solid electrolyte, short-circuiting the cell. Our current understanding of dendrite propagation derives largely from symmetric Li−Li cells with millimetre-thick electrolyte pellets, whereas practical cells will employ solid electrolyte separators less than 100 μm in thickness. In this work, we use in situ synchrotron X-ray computed tomography to show that dendrites in a thin Li6PS5Cl separator, which generally propagate from the lithium anode to the cathode, can be made to propagate laterally by the cathode. Volume contraction of the NMC cathode during charge places the thin separator under lateral compressive stress, deflecting dendritic cracks to propagate laterally (parallel to the electrodes) rather than transversely (across the separator). The result is that the cell resists short circuit even after passing 3 mA h cm−2 capacity at a rate of 3 mA cm−2. Replacing NMC with zero-volume-change pre-lithiated Li4Ti5O12 removes this compressive stress, and the cell rapidly fails by transverse cracking. Chemomechanical modelling rationalises both behaviours, with cathode volume change emerging as a design parameter that can suppress or accelerate failure, with direct implications for cathode selection and cathode/separator interface engineering in practical solid-state batteries.